1. Field of the Invention
The present invention relates to a brazing filler metal superior in corrosion resistance and heat resistance. More particularly, the present invention relates to a brazing filler metal superior in resistance to corrosion by sulfuric acid, which is used for a component of the EGR system (such as EGR cooler). The present invention relates also to an EGR cooler brazed with said brazing filler metal.
2. Description of the Prior Art
Among components requiring high corrosion resistance and heat resistance is that of the EGR (Exhaust Gas Recirculation) system. This system is designed to take out part of exhaust gas from the exhaust system and return it to the induction system of the engine, thereby adding it to mixture. EGR is effective in suppressing the formation of nitrogen oxides, reducing the pumping loss, decreasing the temperature of combustion gas, thereby reducing radiation heat which escapes to the coolant, and increasing the specific heat due to change in amount and composition of the working gas, thereby improving the cycling efficiency. Therefore, it is regarded as an effective means to improve the thermal efficiency of an engine while purifying exhaust gas.
The EGR system mentioned above consists of such components as EGR pipe and EGR valve. Unfortunately, the EGR valve is subject to thermal deterioration (leading to early breakage) as the EGR gas increases in temperature and volume. Coping with this needs a water-cooled structure. In addition, EGR increases the intake temperature and hence decreases the charging efficiency, which leads to a decrease in fuel efficiency. In order to get around these difficulties, there has been developed a cooled EGR system which is so designed as to cool EGR gas with the engine coolant. The cooled EGR system is equipped with an EGR cooler which is a multi-tubular heat exchanger made of austenitic stainless steel (Japanese Patent Laid-open No. 89491/1997) or a plate-type heat exchanger (Japanese Patent Laid-open No. 89880/1998).
Meanwhile, the component (particularly the EGR cooler) of the cooled EGR system is assembled conventionally by brazing with a brazing filler metal designated as BNi-5 in JIS Z3265. BNi-5 is composed of Cr (18-19 wt %), Si (9.5-10.5 wt %), B (0.03 wt % or less), C (0.10 wt % or less), P (0.02 wt % or less), and Ni (remainder). This brazing filler material, however, suffers the disadvantage of having a high melting point of about 1080 to 1135.degree. C. It needs a brazing temperature as high as 1150-1205.degree. C. Brazing at such a high temperature deteriorates the base metal and also produces an adverse effect on corrosion resistance due to precipitation of chromium carbide. This created demand for a brazing filler metal capable of brazing at a lower temperature than before. This demand has been met by the one designated as BNi-2 in JIS Z3265. BNi-2 is composed of Cr (6.0-8.0 wt %), B (2.75-3.50 wt %), Si (4.0-5.0 wt %), Fe (2.5-3.5 wt %), C (0.06 wt % or less), P (0.02 wt % or less), and Ni (remainder). It has a melting point of about 970-1000.degree. C. and is capable of brazing at 1010-1175.degree. C.
BNi-2 is still unsatisfactory because it is poor in resistance to sulfuric acid. Exhaust gas being circulated in the EGR system condenses upon cooling, and the resulting condensate contains sulfuric acid which causes corrosion to the brazing filler metal itself and its diffused part.
Among brazing filler metals specified in JIS Z3265, BNi-7 is the lowest in melting point. It is composed of Cr (13.0-15.0 wt %), B (0.01 wt % or less), Si (0.10 wt % or less), Fe (0.20 wt % or less), C (0.08 wt % or less), P (9.7-10.5 wt %), and Ni (remainder). It has a melting point of 890.degree. C., which is lower than that of BNi-2; however, it is poor in resistance to corrosion by sulfuric acid.